EGR constructions for opposed-piston engines

ABSTRACT

A two-stroke, opposed-piston engine with one or more ported cylinders and uniflow scavenging includes an exhaust gas recirculation (EGR) construction that provides a portion of the exhaust gasses produced by the engine for mixture with charge air to control the production of NOx during combustion.

PRIORITY AND RELATED APPLICATIONS

This application claims priority to US provisional application forpatent 61/395,845 filed May 18, 2010, and to U.S. provisionalapplication for patent 61/401,598 filed Aug. 16, 2010.

BACKGROUND

The field is internal combustion engines. Particularly, the fieldrelates to ported, uniflow-scavenged, opposed-piston engines withexhaust gas recirculation. More particularly, the field includestwo-stroke, opposed-piston engines with one or more ported cylinders anduniflow scavenging in which an exhaust gas recirculation (EGR)construction provides a portion of the exhaust gasses produced by theengine for mixture with charge air to control the production of NOxduring combustion.

As seen in FIG. 1, an internal combustion engine is illustrated by wayof an opposed-piston engine that includes at least one cylinder 10 witha bore 12 and longitudinally-displaced exhaust and intake ports 14 and16 machined or formed therein. Fuel injector nozzles 17 are located inor adjacent injector ports that open through the side of the cylinder,at or near the longitudinal center of the cylinder. Two pistons 20, 22are disposed in the bore 12 with their end surfaces 20 e, 22 e inopposition to each other. For convenience, the piston 20 is referred asthe “exhaust” piston because of its proximity to the exhaust port 14;and, the end of the cylinder wherein the exhaust port is formed isreferred to as the “exhaust end”. Similarly, the piston 22 is referredas the “intake” piston because of its proximity to the intake port 16,and the corresponding end of the cylinder is the “intake end”.

Operation of an opposed-piston engine with one or more cylinders such asthe cylinder 10 is well understood. In this regard, and with referenceto FIG. 2, in response to combustion occurring between the end surfaces20 e, 22 e the opposed pistons move away from respective top dead center(TDC) positions where they are at their closest positions relative toone another in the cylinder. While moving from TDC, the pistons keeptheir associated ports closed until they approach respective bottom deadcenter (BDC) positions in which they are furthest apart from each other.The pistons may move in phase so that the exhaust and intake ports 14,16 open and close in unison. Alternatively, one piston may lead theother in phase, in which case the intake and exhaust ports havedifferent opening and closing times.

In many opposed-piston constructions, a phase offset is introduced intothe piston movements. As shown in FIG. 1, for example, the exhaustpiston leads the intake piston and the phase offset causes the pistonsto move around their BDC positions in a sequence in which the exhaustport 14 opens as the exhaust piston 20 moves through BDC while theintake port 16 is still closed so that combustion gasses start to flowout of the exhaust port 14. As the pistons continue moving away fromeach other, the intake port 16 opens while the exhaust port 14 is stillopen and a charge of pressurized air (“charge air”) is forced into thecylinder 10, driving exhaust gasses out of the exhaust port 14. Thedisplacement of exhaust gas from the cylinder through the exhaust portwhile admitting charge air through the intake port is referred to as“scavenging”. Because the charge air entering the cylinder flows in thesame direction as the outflow of exhaust gas (toward the exhaust port),the scavenging process is referred to as “uniflow scavenging”.

As the pistons move through their BDC locations and reverse direction,the exhaust port 14 is closed by the exhaust piston 20 and scavengingceases. The intake port 16 remains open while the intake piston 22continues to move away from BDC. As the pistons continue moving towardTDC (FIG. 2), the intake port 16 is closed and the charge air in thecylinder is compressed between the end surfaces 20 e and 22 e.Typically, the charge air is swirled as it passes through the intakeport 16 to promote good scavenging while the ports are open and, afterthe ports close, to mix the air with the injected fuel. Typically, thefuel is diesel which is injected into the cylinder by high pressureinjectors. With reference to FIG. 1 as an example, the swirling air (orsimply, “swirl”) 30 has a generally helical motion that forms a vortexin the bore which circulates around the longitudinal axis of thecylinder. As best seen in FIG. 2, as the pistons advance toward theirrespective TDC locations in the cylinder bore, fuel 40 is injectedthrough the nozzles 17 directly into the swirling charge air 30 in thebore 12, between the end surfaces 20 e, 22 e of the pistons. Theswirling mixture of charge air and fuel is compressed in a combustionchamber 32 defined between the end surfaces 20 e and 22 e when thepistons 20 and 22 are near their respective TDC locations. When themixture reaches an ignition temperature, the fuel ignites in thecombustion chamber, driving the pistons apart toward their respectiveBDC locations.

As illustrated in FIG. 2, fuel is directly injected through the side ofthe cylinder (“direct side injection”) into the cylinder bore and themovement of the fuel interacts with the residual swirling motion of thecharge air in the bore. As the engine operating level increases and theheat of combustion rises, an increasing amount of nitrogen oxide (NOx)is produced. However, increasingly stringent emission requirementsindicate the need for a significant degree of NOx reduction. Onetechnique reduces NOx emission by exhaust gas recirculation (“EGR”). EGRhas been incorporated into spark-ignited 4-stroke engine constructionsand 2-stroke, compression-ignition engines with a single pistonoperating in each cylinder. However, such EGR constructions are notimmediately applicable to 2-stroke, opposed-piston engines with uniflowscavenging because of the need to generate a pressure differential thatpumps the exhaust gas into the inflowing stream of air in anopposed-piston engine. Therefore, there is a need for effective EGRconstructions that are adapted to the designs and operations of 2-strokeopposed-piston engines with uniflow scavenging.

SUMMARY

A solution to the problem is to reduce the NOx emissions of a two-strokeopposed-piston engine with uniflow scavenging by exhaust gasrecirculation through one or more ported cylinders of the engine. Theengine includes at least one cylinder with piston-controlled exhaust andintake ports and a charge air channel to provide charge air to at leastone intake port of the engine.

In one aspect, EGR is provided by an EGR loop having an input coupled toan exhaust port of the cylinder and a loop output coupled to the chargeair channel. A pressure differential provided between the exhaust gasand the charge air channel causes the exhaust gas to flow through theEGR loop to the charge air channel where exhaust gas and air are mixedand provided to the at least one intake port.

In another aspect, EGR is provided by retention of residual exhaustgasses in the ported cylinder when scavenging ceases.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side sectional partially schematic drawing of a cylinder ofa prior art opposed-piston engine with opposed pistons near respectivebottom dead center locations, and is appropriately labeled “Prior Art”.

FIG. 2 is a side sectional partially schematic drawing of the cylinderof FIG. 1 with the opposed pistons near respective top dead centerlocations where end surfaces of the pistons define a combustion chamber,and is appropriately labeled “Prior Art”.

FIG. 3 is a conceptual schematic diagram of an internal combustionengine of the opposed-piston type in which aspects of an air managementsystem with EGR are illustrated.

FIG. 4 is a conceptual schematic drawing illustrating preferredconstructions for EGR in the ported, uniflow scavenging, internalcombustion engine of FIG. 3.

FIG. 5 is a schematic drawing of a preferred EGR construction for theported, uniflow scavenging, internal combustion engine of FIG. 3.

FIG. 6 is a schematic drawing of an alternate EGR construction for aported, uniflow scavenging, opposed-piston engine without aturbocharger.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The EGR constructions described in this specification are presented inan explanatory context that includes a ported, uniflow-scavenginginternal combustion engine having at least one cylinder in which a pairof pistons is disposed with their end surfaces in opposition. A “ported”cylinder includes one or more of intake and exhaust ports formed ormachined in a sidewall thereof. This explanatory context is intended toprovide a basis for understanding various EGR construction embodimentsby way of illustrative examples.

In FIG. 3, an internal combustion engine 49 is embodied by anopposed-piston engine having at least one ported cylinder 50. Forexample, the engine may have one ported cylinder, two ported cylinders,three ported cylinders, or four or more ported cylinders. For purposesof illustration, in the examples to be illustrated and described theengine is an engine of the opposed-piston type that is presumed to havea plurality of ported cylinders. In this regard, each cylinder 50 has abore 52 and exhaust and intake ports 54 and 56 formed or machined inrespective ends thereof. The exhaust and intake ports 54 and 56 eachinclude one or more circumferential arrays of openings in which adjacentopenings are separated by a solid bridge. (In some descriptions, eachopening is referred to as a “port”; however, the construction of acircumferential array of such “ports” is no different than the portconstructions shown in FIG. 3.) Exhaust and intake pistons 60 and 62 areslidably disposed in the bore 52 with their end surfaces 61 and 63opposing one another. The exhaust pistons 60 are coupled to a crankshaft71, the intake pistons are coupled to the crankshaft 72.

When the pistons 60 and 62 of a cylinder 50 are at or near their TDCpositions, a combustion chamber is defined in the bore 52 between theend surfaces 61 and 63 of the pistons. Fuel is injected directly intothe combustion chamber through at least one fuel injector nozzle 100positioned in an opening through the sidewall of a cylinder 50.

With further reference to FIG. 3, the engine 49 includes an airmanagement system 51 that manages the transport of charge air providedto, and exhaust gas produced by, the engine 49. A representative airmanagement system construction includes a charge air subsystem and anexhaust subsystem. In the air management system 51, the charge airsubsystem includes a charge air source that receives intake air andprocesses it into charge air, a charge air channel coupled to the chargeair source through which charge air is transported to the at least oneintake port of the engine, and at least one air cooler in the charge airchannel that is coupled to receive and cool the charge air (or a mixtureof gasses including charge air) before delivery to the intake port orports of the engine. Such a cooler can comprise an air-to-liquid and/oran air-to-air device, or another cooling device. Hereinafter, such acooler is denoted as a “charge air cooler”. The charge air subsystemalso includes a supercharger that pumps charge air in the charge airchannel to intake ports of the engine. The exhaust subsystem includes anexhaust channel that transports exhaust products from exhaust ports ofthe engine to an exhaust pipe.

With reference to FIG. 3, the preferred charge air subsystem includes asupercharger 110, which can be driven by an electrical motor, or by agear, chain, or belt apparatus coupled to a crankshaft. For example, butwithout limitation, in FIGS. 4, 5, and 6 the supercharger 110 is coupledby a belt linkage to the crankshaft 72 to be driven thereby. Thesupercharger 110 can be a single-speed or multiple-speed device, or afully variable-speed device. Preferably, but not necessarily, the airmanagement system 51 includes a turbo-charger 120 with a turbine 121 anda compressor that rotate on a common shaft 123. The turbine 121 iscoupled to the exhaust subsystem and the compressor 122 is coupled tothe charge air subsystem. The turbine 121 can be a fixed-geometry or avariable-geometry device. The turbo-charger 120 extracts energy fromexhaust gas that exits the exhaust ports 54 and flows into the exhaustchannel 124 directly from the exhaust ports 54, or from an exhaustmanifold 125. In this regard, the turbine 121 is rotated by exhaust gaspassing through it. This rotates the compressor 122, causing it togenerate charge air by compressing intake air. The charge air output bythe compressor 122 flows through a conduit 126 to a charge air cooler127, whence it is pumped by the supercharger 110 to the intake ports.Air compressed by the supercharger 110 is output from the superchargerthrough a charge air cooler 129 to an intake manifold 130. The intakeports 56 receive charge air pumped by the supercharger 110, through theintake manifold 130. Preferably, but not necessarily, in multi-cylinderopposed-piston engines, the intake manifold 130 is constituted of anintake plenum that communicates with the intake ports 56 of allcylinders 50.

Exhaust Gas Management: It is desirable to modify or adapt an airmanagement construction for an internal combustion engine of theported-cylinder type in order to reduce NOx emissions produced bycombustion. It is particularly desirable to control such emissions byrecirculating exhaust gas through the ported cylinders of anopposed-piston engine. The recirculated exhaust gas is mixed with chargeair to lower peak combustion temperatures, which lowers NOx emissions.This process is referred to as exhaust gas recirculation (“EGR”). An EGRconstruction can utilize exhaust gasses transported in an EGR channelexternal to the cylinder into the incoming stream of fresh intake air asper the valve-controlled recirculation channel 131 in FIG. 3.Alternatively, or additionally, an EGR process can utilize residualexhaust gasses that are retained in the cylinders 50 when scavengingceases. In the case of external EGR, the exhaust gas is pumped into theinflowing stream of air. A source of pressure in communication with theEGR channel creates a pressure differential that causes exhaust gas toflow through the EGR channel into the charge air subsystem. In someaspects, a virtual pump exists when the exhaust gas to be recirculatedis obtained from a source guaranteed to be at a higher pressure than thepoint where it is fed into the intake stream of charge air. In otheraspects, an active pump, such as the supercharger 110, is used to pumpthe exhaust gas to be recirculated into the charge air that thesupercharger is pumping to the intake ports. In these aspects, use of asupercharger provides an additional variable for controlling EGRoperations in an opposed-piston engine. In some aspects recirculatedexhaust gas is cooled by way of one or more EGR coolers, which cancomprise air-to-liquid and/or air-to-air devices. In other aspects,recirculated exhaust gas is cooled by one or more charge air coolersalone or in combination with one or more EGR coolers.

First EGR loop construction: In some aspects, the internal combustionengine seen in FIG. 3 includes a first EGR loop construction. Withreference to FIG. 4, a first EGR loop construction for a uniflowscavenging, ported, opposed-piston application circulates exhaust gasfrom any source of exhaust gas exiting from one or more cylinders. Forexample, but without excluding other exhaust gas sources, the first EGRloop construction includes an EGR port 55 positioned inboard of exhaustport 54; that is to say, the EGR port 55 is positioned between theexhaust port 54 and the longitudinal midpoint of the cylinder 50. TheEGR port construction includes one or more port openings as needed byany particular design. In response to combustion, while moving towardBDC, the exhaust piston 60 moves past the EGR port 55, opening the EGRport to a cylinder bore pressure that is guaranteed to be higher thanthe pressure in the intake manifold 130. This pressure differentialpumps a portion of exhaust gas from the EGR port 55 through a plenum ormanifold (not shown) into a conduit 133 controlled by a one-way checkvalve 134, and then into the intake manifold 130 where it is mixed withthe charge air and recirculated therewith into the cylinder bore.Preferably, but not necessarily, the exhaust gas enters the charge airoutput by the supercharger 110 prior to the inlet of the charge aircooler 129. It is desirable that the ratio of the cylinder pressure whenthe EGR port opens (“EGR open”) to the intake manifold pressure notexceed a threshold value beyond which choked flow conditions may occurin the one-way check valve 134. This pressure ratio is affected by thesize of the EGR port and its location with respect to the longitudinalcenter of the cylinder (the closer to the center, the higher thepressure) as well as the state of the air charge system (boost, turbineback pressure, etc.).

With further reference to FIG. 4, in a variant of the first loop EGRconstruction, the exhaust gas flowing in the conduit 133 is mixed withcharge air output by the supercharger 110 through a mixer 135 that canbe constituted, for example, as a venturi. The exhaust gas is input tothe mixer 135 through a valve 136; pressurized charge air output by thesupercharger 110 is provided to a mixing input of the mixer 135. Themixture of pressurized charge air and exhaust gas produced by the mixer135 is provided to the input of the charge air cooler 129 (or,alternately, to the input of the charge air cooler 127). The valve 136is operated by a signal output by an engine control unit (ECU) 149.

In some, aspects it is desirable to dampen fluctuations in the flow ofexhaust gas. In such a case and with reference to FIG. 4, an accumulator145 is provided in the first loop in series between the EGR port 55 andthe input to the valve 136. In some other aspects, it is desirable tocool the exhaust gas before mixing it with charge air. In such a case,an EGR cooler 146 is provided in the first loop in series between thecheck valve 134 and the input to the valve 136. Alternatively, the loopconstruction can be 134, 136, 146. In the case where both the EGRaccumulator 145 and the cooler 146 are used, it is preferred, but notrequired, that the EGR cooler 146 be positioned in series between theoutput of the EGR accumulator 145 and the input to the valve 136.

Second EGR loop construction: In some aspects, the internal combustionengine 49 seen in FIG. 3 can include another EGR loop construction. Withreference to FIGS. 3 and 4, a second EGR loop construction includes theconduit 131 and a valve 138 to shunt a portion of the exhaust gas fromthe exhaust manifold 54 to the input of a charge air cooler so that theportion is cooled. In order to promote optimal exhaust gas/charge airmixing, it is desirable to add a device through which the exhaust gassesand the charge air flow together and mix. When it is desirable tointroduce the exhaust gas into the charge air at a location that isremote from the intake manifold 56, the exhaust gas portion is shuntedto the input of the charge air cooler 127. This loop subjects theexhaust gas to the cooling effects of two charge air coolers (127 and129). If less cooling is merited, the valve 138 can be constituted as athree-way valve (as best seen in FIG. 4), and the exhaust gas portioncan be shunted around the cooler 127 to the input of the supercharger110. This alternative subjects the exhaust gas portion to cooling byonly the charge air cooler 129. A dedicated EGR cooler that cools onlyexhaust gas can be incorporated into the second loop, if needed. Forexample, an EGR cooler can be placed in the conduit 131, in series withthe valve 138, or in series with the output port of the valve 138 andthe input to the supercharger 110. In some aspects, the valve 138 isconstituted as a single three-way device. Alternatively, the valve 138is constituted as a pair of valves, each in a respective branch of aY-connection from the conduit 131, in which one valve controls theprovision of exhaust gas to the input of the cooler 127 and the othercontrols the provision of exhaust gas to the input of the supercharger110.

EGR using retained exhaust gas: In a uniflow or loop-scavenged internalcombustion engine, it is sometimes desirable to trap or retain aresidual amount of exhaust gas in a cylinder after scavenging ceases.The residual exhaust gas can be used to adjust the initial conditionsfor combustion to a point advantageous for reducing NOx emissions.Depending on the configuration of the turbo-machinery, at low and mediumspeeds and loads, uniflow-scavenged engines may exhibit incompletescavenging. Since the residual exhaust gas inside the cylinder is hot,the resulting temperature of the new charge of air may be substantiallyelevated, therefore this method is best suited for reducing NOx underpartial engine load conditions.

The amount of charge air that is fed into a cylinder each cycle can beused to alter the amount of residual exhaust gas left in the cylinder.In this regard, adjusting the amount of charge air that is fed into thecylinder in any given cycle of operation can be used to “tune” theamount of exhaust gas retained in the cylinder for the next combustionoccurrence. In one aspect of retained exhaust gas EGR, seen in FIG. 4, abypass conduit loop 148 including a valve 139 is placed in parallel withthe supercharger 110. The valve 139 is operated to control the amount ofcharge air pumped into the engine by the supercharger 110. Setting theamount of charge air pumped allows control of the amount of exhaust gasscavenged, and, consequently, the amount of exhaust gas retained in anycylinder following scavenging. In this regard, if a high manifoldpressure is desired (as would be indicated for high engine loadconditions) the valve 139 is fully shut and charge air is delivered tothe engine at a high rate. As the valve 139 is increasingly opened, anincreasing amount of charge air pumped by the supercharger 110 isreturned to the inlet of the supercharger, which proportionately reducesthe amount of charge air delivered to the engine. Thus, the chargeair/fuel ratio is reduced and the amount of exhaust gas retained in anycylinder is increased. Among the benefits realized by this aspect ofretained exhaust gas EGR are NOx reduction and reduction of pumping loadimposed on the engine by the supercharger 110.

An increase in the pressure felt by exhaust gas flowing to the turbine(“backpressure”) can also be used to alter the amount of residualexhaust gas left in the cylinder. In this regard, adjusting the amountof backpressure in any given cycle of operation can be used to “tune”the amount of residual exhaust gas for the next combustion occurrence.Therefore, in another aspect of retained exhaust gas EGR, seen in FIG.4, a variable valve 140 is placed in series with exhaust gas output. Thesetting of the valve 140 directly influences the backpressure feltupstream of the valve and, consequently, the amount of exhaust gasretained in any cylinder after scavenging. In FIG. 4, the valve 140 isplaced in series with the output of the turbine 121. In this case, anybackpressure resulting from the setting of the valve is distributed overall cylinders of the engine. In an alternative aspect, an equivalentvalve 140 a can be placed in series between the input to the turbine 121and an exhaust manifold that collects the exhaust output of one or morecylinders. In yet another alternative aspect, the equivalent valve 140 acan be placed in series with an exhaust manifold or exhaust port of eachof a plurality of cylinders.

Turbine bypass construction: Referring again to FIG. 4, a bypass conduitloop 143 including a valve 144 is placed in parallel with the turbine121. The valve 144 is operated to control the amount of exhaust gasflowing from the engine into the turbine 121. Setting the valve 144 tobypass the turbine 121 allows exhaust energy to be dumped into theexhaust pipe 128 without operating the turbine 121 and compressor 122.This keeps the exhaust gas at a higher temperature level and increasesafter-treatment conversion efficiencies (for particulate filters andcatalytic devices, for example) at engine warm-up during partial engineload conditions such as from a cold start. Further, setting the valve144 to bypass the turbine 121 during engine operation under partialengine load conditions reduces turbo-charger operation, and allows moreexhaust gas to be driven over the supercharger 110 (via valve 138, forexample) while also delivering exhaust gas at a higher temperature tothe exhaust pipe 128 to increase after-treatment conversionefficiencies. Another construction for varying the amount of exhaust gasflowing from the engine into the turbine 121 includes a turbine with avariable geometry construction to control pressure in the exhaustconduit 124, upstream of the valve 144. Using a variable geometryturbine (VGT) instead of a fixed geometry turbine does not necessarilyeliminate the need for a turbine bypass valve such as the valve 144. AVGT has only a limited mass flow range where it works at acceptableefficiencies. Outside this range, a turbine bypass valve can be used tocontrol the mass flow and intake pressure of the engine 49.

Preferred EGR Embodiment: A preferred EGR construction for a portedopposed-piston engine with uniflow scavenging is shown in FIG. 5. In thepreferred construction, exhaust gas flows from the exhaust port or ports54 of the engine, through the conduit 124 to the turbine 121, whence itpasses through after-treatment conversion (not shown) and flows out theexhaust pipe 128. Preceding the input to the turbine 121, a portion ofthe exhaust gas is shunted from the conduit 124 via 131 and from therethrough the valve 138′ to the input of the charge air cooler 127 whereit mixes with the incoming stream of fresh air. The exhaust gas and airare mixed and cooled in the charge air cooler 127, and the cooledgas/air mixture is input to the supercharger 110. The supercharger 110compresses the gas/air mixture, and the compressed mixture is input tothe charge air cooler 129. The cooled, compressed mixture then entersthe cylinder 50 via the intake port 56. Optionally, the intake throttlevalve 141 and the turbine bypass valve 144 are included for highprecision control of the ratio of recirculated exhaust gas to fresh air.

Exhaust configuration and control: The EGR and turbine bypassconstructions illustrated in FIGS. 4 and 5 can be implemented in aported engine of the uniflow scavenging type singly, or in anycombination of two or more constructions, or portions thereof, asrequired for a specific design. One example is an EGR configuration inwhich uncooled exhaust gas retained in a cylinder following scavengingis combined or mixed with recirculated exhaust gas that is cooled andmixed with charge air provided to the cylinder. The relative amounts ofretained and recirculated exhaust gas can be varied in order toprecisely control the EGR rate and temperature. An intake throttle valve141 can be placed in the stream of fresh air flowing into the compressor122 in order to more precisely control the ratio of recirculated exhaustgas to fresh air. If implemented on a per-cylinder basis, a high-speedindividual EGR and charge air/fuel trim is provided to correctcylinder-to-cylinder variations caused by flow dynamics and/ormanufacturing tolerances.

An EGR control process for an EGR system that utilizes one or more ofthe constructions illustrated in FIGS. 4 and 5, or any combinationthereof, is executed by the ECU 149 in response to specified engineoperating conditions by automatically operating any one or more of thevalves 136, 138, 139, 140, 140 a, and 144, the intake throttle valve141, and the supercharger 110, if a multi-speed or variable speed deviceis used, and the turbo-charger 120, if a variable-geometry device isused. Of course, operation of valves, throttles, and associated elementsused for EGR can include any one or more of electrical, pneumatic,mechanical, and hydraulic actuating operations. For fast, preciseautomatic operation, it is preferred that the valves be high-speed,computer-controlled devices with continuously-variable settings. Eachvalve has a first state in which it is open (to some setting controlledby the ECU 149) to allow gas to flow through it, and a second state inwhich it is closed to block gas from flowing through it.

Preferably an EGR control process automatically operates an EGR systemincorporating one or more constructions described and illustrated hereinbased upon one or more parameters relating to recirculated exhaust gasand to a mixture of recirculated exhaust gas and charge air. Parametervalues are determined by a combination of one or more of sensors,calculations, and table lookup so as to manage the values of individualparameters and one or more ratios of EGR and mixture parameters in oneor more cylinders.

Alternate EGR Embodiment: An alternate EGR construction is shown in FIG.6 in a two-stroke opposed-piston engine with ported cylinders anduniflow scavenging in which only a supercharger provides scavengingpressure. Presume that after-treatment conversion is implemented byemission control devices that include a diesel oxidation catalyst (DOC)to reduce CO and hydrocarbons, a diesel particulate filter (DPF) toreduce soot emissions and a selective catalytic reduction device toreduce NOx emissions. All of these devices require addition of heat foroperation, and the absence of a turbocharger reduces the competition forheat derived from exhaust gas, while also lowering the power density ofthe engine. Moreover, the DPF and DOC now can be closely coupled at anexhaust manifold, where a turbocharger typically is mounted.Furthermore, elimination of a turbocharger and its required ductingreduces the size of the opposed-piston engine and also reduces the lossof exhaust heat by convection from the turbocharger housing and ducting.Preferably, although not necessarily, exhaust gas for recirculation isextracted from the outlet of the DPF where it is free of particulatesand can be cooled and plumbed to the inlet of the supercharger. Althoughthe exhaust gas is cooler after the DPF, it can be cooled further withan EGR cooler.

The alternate EGR embodiment for a two-stroke, ported,uniflow-scavenging, opposed-piston engine is illustrated in FIG. 6.Preferably, the engine does not include a turbocharger. Exhaust gasflows from the exhaust manifold 125, through the conduit 124, throughthe DOC 150 and the DPF 151, then through the valve 140 and out theexhaust pipe 128. A portion of the exhaust gas is diverted by a pressurechange determined by the setting of the valve 140 into the input of anEGR cooler 142. Cooled exhaust gas output by the EGR cooler 142 ismetered through a valve 147 into the air stream entering thesupercharger 110. The intake throttle valve 141 can be placed in theairstream flowing to the supercharger, upstream of the output of thevalve 147, in order to more precisely control the ratio of recirculatedexhaust gas to air by creating a slight vacuum. Since the alternate EGRloop is being drawn through the supercharger 110, the time needed toempty the exhaust gas from the charge air cooler 129 is much reduced,thereby improving the transient response. If the supercharger 110 isdriven directly from the engine, it will achieve high flow and highspeed together with the engine. The supercharger capacity enables theneeded exhaust gas to be pumped as required at high engine speed andload as required to meet stringent emission requirements. Thesupercharger bypass valve 139 permits the pressure produced by thesupercharger to be continuously varied.

Although EGR constructions have been described with reference to aported opposed engine construction with two crankshafts, it should beunderstood that various aspects of these constructions can be applied toopposed-piston engines with one or more crankshafts. Moreover, variousaspects of these EGR constructions can be applied to opposed-pistonengines with ported cylinders disposed in opposition, and/or on eitherside of one or more crankshafts. Accordingly, the protection afforded tothese constructions is limited only by the following claims.

1. A ported, uniflow-scavenged, opposed-piston engine including at leastone cylinder with piston-controlled exhaust and intake ports and acharge air channel to provide charge air to at least one intake port ofthe engine, in which an exhaust gas recirculation (EGR) loop has a loopinput coupled to an exhaust port of the cylinder and a loop outputcoupled to the charge air channel, and the engine includes a pump incommunication with the EGR loop to pump exhaust gas through the EGR loopinto the charge air channel.
 2. The ported, uniflow-scavenged,opposed-piston engine of claim 1, in which the charge air channelincludes at least one charge air cooler, wherein the loop output iscoupled in series with the at least one charge air cooler.
 3. Theported, uniflow-scavenged, opposed-piston engine of claim 2, in whichthe EGR loop includes a variable valve settable to a first state or to asecond state, wherein the first state couples the loop output to acharge air input of the at least one charge air cooler and the secondstate couples the loop output to a charge air output of the at least onecharge air cooler.
 4. The ported, uniflow-scavenged, opposed-pistonengine of claim 3, in which the pump includes a supercharger and the atleast one charge air cooler includes a charge air cooler with an inputcoupled to the compressor output of a turbo-charger and an outputcoupled to the input of the supercharger, in which the first statecouples the loop output to the input of the charge air cooler and thesecond state couples the loop output to the input of the supercharger.5. The ported, uniflow-scavenged, opposed-piston engine of claim 2,further including a charge air source with a charge air output coupledto the input of the at least one charge air cooler, in which the loopoutput includes a valve settable to a first state or to a second stateand a mixer with an exhaust gas input, a charge air input, and a mixeroutput coupled to the input of the at least one charge air cooler, inwhich the first: state of the valve couples the loop output to theexhaust gas input of the mixer and the second state of the valveuncouples the loop output from the exhaust gas input of the mixer. 6.The ported, uniflow-scavenged, opposed-piston engine of claim 2, furtherincluding a supercharger with a supercharger input and a charge airoutput coupled to the charge air input of the at least one charge aircooler and a valve in parallel with the supercharger, in which the valveis settable to a first state in which the supercharger input is coupledthrough the valve to the charge air output of the supercharger and to asecond state in which the supercharger input is uncoupled through thevalve from the charge air output of the supercharger.
 7. The ported,uniflow-scavenged, opposed-piston engine of claim 1, further including aturbo-charger with a charge air output coupled to the charge air channeland a turbine input coupled to the exhaust port, and a back pressurevalve in series between the turbine input and the exhaust port, in whichthe backpressure valve is settable to a state causing a back pressureacting upon the exhaust port.
 8. The ported, uniflow-scavenged,opposed-piston engine of claim 7, in which the turbo-charger includes avariable-geometry turbine.
 9. The ported, uniflow-scavenged,opposed-piston engine of claim 1, further including a turbo-charger witha charge air output coupled to the charge air channel, a turbine inputcoupled to the exhaust port, a turbine output coupled to an exhaustoutput, and a back pressure valve in series between the turbine outputand the exhaust output, in which the backpressure valve is settable to astate causing a back pressure acting upon the exhaust port.
 10. Theported, uniflow-scavenged, opposed-piston engine of claim 9, in whichthe turbo-charger includes a variable-geometry turbine.
 11. The ported,uniflow-scavenged, opposed-piston engine of claim 1, further including aturbo-charger with a charge air output coupled to the charge air channeland a turbine with an input coupled to the exhaust port and a turbinebypass valve in parallel with the turbine, in which the turbine bypassvalve is settable to a first state in which the turbine input is coupledthrough the valve to a turbine output and to a second state in which theturbine input is uncoupled through the valve from the turbine output.12. The ported, uniflow-scavenged, opposed-piston engine of claim 11, inwhich the turbo-charger includes a variable-geometry turbine.
 13. Aported, uniflow-scavenged, opposed-piston engine including at least onecylinder with piston-controlled exhaust and intake ports and a chargeair channel coupled to at least one intake port of the engine, in whichan exhaust gas recirculation (EGR) loop has a loop input coupled to anEGR port of the cylinder located between the exhaust port and thelongitudinal center of the cylinder, and a loop output coupled to thecharge air channel.
 14. The ported, uniflow-scavenged, opposed-pistonengine of claim 13, in which the EGR loop includes a valve having aninput coupled to the loop input, and an output coupled to a mixer in thecharge air channel.
 15. The ported, uniflow-scavenged, opposed-pistonengine of claim 14, in which the EGR loop further includes an EGR coolerin series with the valve.
 16. A method of operating a ported,uniflow-scavenged, opposed-piston engine with one or more cylinders, inwhich charge air is pressurized and then cooled in at least one chargeair cooler and provided to an intake port of each of the one or morecylinders, by providing engine exhaust gas through an exhaust gasrecirculation (EGR) loop to an input of the at least one air chargecooler.
 17. The method of operating a ported, uniflow-scavenged,opposed-piston engine of claim 16, further including retaining exhaustgas in the one or more cylinders during low engine load conditions. 18.The method of operating a ported, uniflow-scavenged, opposed-pistonengine of claim 17, in which exhaust gas is retained in the one or morecylinders by decreasing charge air pressure.
 19. The method of operatinga ported, uniflow-scavenged, opposed-piston engine of claim 17, in whichexhaust gas is retained in the one or more cylinders by increasingexhaust gas back pressure.
 20. The method of operating a ported,uniflow-scavenged, opposed-piston engine of claim 17 in which exhaustgas is further retained in the one or more cylinders by a combination ofincreasing exhaust gas back pressure and decreasing charge air pressure.21. A ported, uniflow-scavenged, opposed-piston engine including atleast one cylinder with piston-controlled exhaust and intake ports and acharge air channel to provide charge air to at least one intake port ofthe engine, in which the charge air channel includes a supercharger withan output and an input coupled to a source of air, a charge air coolerwith an input coupled to the supercharger output and an output coupledto at least one intake port, an exhaust channel includes at least oneheat-actuated exhaust treatment device with an input coupled to at leastone exhaust port and an output, and an exhaust gas recirculation (EGR)loop with a loop input coupled to an output of the at least oneheat-actuated exhaust treatment device and a loop output coupled to theinput of the supercharger.
 22. The ported, uniflow-scavenged,opposed-piston engine of claim 21, in which the EGR loop includes acooler with an input coupled to the output of the at least oneheat-actuated exhaust treatment device and an output, and a variablevalve settable to a first state or to a second state, wherein the firststate decouples the loop output from the supercharger input and thesecond state couples the loop output to the supercharger input.
 23. Theported, uniflow-scavenged, opposed-piston engine of claim 22, in whichthe source of air includes an intake throttle valve for controlling aratio of recirculated exhaust gas provided by the EGR loop to airprovided by the source of air.
 24. The ported, uniflow-scavenged,opposed-piston engine of claim 23 without a turbocharger.
 25. A methodof operating a ported, uniflow-scavenged, opposed-piston engine with oneor more cylinders in which a stream of intake air is supercharged andthen cooled in a charge air cooler, by providing cooled, superchargedair to at least one intake port, recirculating exhaust gas output from aheat-operated exhaust treatment device, cooling the recirculated exhaustgas, introducing the cooled, recirculated exhaust gas into the stream ofintake air, and controlling a ratio of cooled, recirculated exhaust gasto fresh air provided by the source of fresh air.
 26. The method ofclaim 25, in which controlling a ratio includes automatically throttlingthe amount of fresh air in the stream of fresh air.
 27. In auniflow-scavenged, opposed-piston engine with one or more portedcylinders and without a turbocharger, an exhaust gas recirculation loopcharacterized by: a first cooler having an input coupled to an output ofa heat-operated exhaust treatment device; a supercharger having an inputcoupled to a source of intake air and an output; a second cooler havingan input coupled to the output of the supercharger and an output coupledto at least one intake port; and, a valve coupled between the output ofthe first cooler and the input of the supercharger.
 28. The exhaust gasrecirculation loop of claim 27, further including a throttle valvehaving an input coupled to the source of intake air and an outputcoupled to the input of the supercharger, in which the valve has aninput coupled to the output of the first cooler and an output coupled incommon with the output of the throttle valve and the input to thesupercharger.